Geoscience Reference
In-Depth Information
180°
0°
(A)
(B)
BG
90°W
90°E
90°W
90°E
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180°
MAXIMUM
ANTARCTIC CONVERGENCE
AVERAGE SEASONAL ICE COVER
MINIMUM
Figure 10.35
A: Surface currents in the Arctic, together with average autumn minimum and spring maximum sea
ice extent for the period 1973-1990. There have been record minima in summary 2005, 2007 and 2008. B: Southern
Ocean surface circulation, convergence zones and seasonal ice limits in March and September.
Sources: A: Maythum (1993) and Barry (1983). B: After Barry (1986). Copyright © Plenum Publishing Corp., New York. Published by
permission.
depressions tend to be similarly located and
generally form, travel and decay within a period
of about a week. As in the Northern Hemisphere,
high zonal index results from a strong meridional
pressure gradient and is associated with wave
disturbances propagated eastward at high speed
with irregular and often violent winds and zonally
oriented fronts. Low zonal index results in high
pressure ridges extending further south and low
pressure centers located further north. However,
breakup of the flow, leading to blocking, is less
common and less persistent in the Southern
Hemisphere than in the Northern.
The southern westerlies are linked to the belt
of traveling anticyclones and troughs by cold
fronts, which connect the inter-anticyclonic
troughs of the latter with the wave depressions of
the former. Although storm tracks of the wester-
lies are usually well to the south of Australia
(
Figure 10.33
), fronts may extend north into the
continent, particularly from May, when the first
rains occur in the southwest. On average, in
midwinter (July), three depression centers skirt
the southwest coast. When a deep depression
moves to the south of New Zealand, the passage
of the cold front causes that country to be covered
first by a warm, moist westerly or northerly
airflow and then by cooler southerly air. A series
of such depressions may follow at intervals of
12-36 hours, each cold front being followed by
progressively colder air. Further east over the
South Pacific, the northern fringe of the southern
westerlies is influenced by northwesterly winds,
changing to west or southwest as depressions
move to the south. This weather pattern is
interrupted by periods of easterly winds if
depression systems track along lower latitudes
than usual.
